Acrylic resin, such as polymethyl methacrylate, is used as a component for optics, such as an optical lens, a light diffusing plate or a light conducting plate, or as an optical film in light of optical properties and the transparency thereof; is used as a cap stock for covering a surface of a matter used outdoors while good use is made of excellent weather resistance thereof; and is used as a film that may be of various types for an alternative to a decoration such as coating or plating, or for the protection of a member.
When acrylic resin is worked into a molded product as described above, there may remain, in the resin, a monomer or oligomer component; additives for resin, such as a stabilizer, or decomposition products originating from a secondary material for polymerization; and others. In this case, the resin is softened and further gas is generated. In injection molding, the gas adheres onto a molding machine or a mold therefor. In this way, dirt adheres onto the mold, and the dirt may adhere again onto a surface of the molded product to result in a poor external appearance of the molded product, or in the incorporation of an alien substance therein. Thus, in a subsequent step, this inconvenience may cause a defect. It is therefore indispensable to wash the facilities (concerned) and the mold periodically. In contour extrusion molding, the generated gas is cooled at an inlet in the calibrator (concerned), so that a precipitation is generated. The accumulated precipitation contacts the molded product surface so that external appearance defects, such as a streak are caused therein. In the molding into a film, gas generated near a die adheres onto the cast roller (concerned), so that the resultant dirt is transferred onto the film to give fatal defects against important film properties themselves, such as a decline in the transparency, and a deterioration in the external appearance. The gas-generation in the molding process makes the quality of the molded body (product) bad, and further causes a deterioration in the working environment. Thus, it is necessary to take countermeasures about which the safety of workers is considered (such as the introduction of a protecting tool, or an exhaust system).
As a method for removing a volatile component in the resin, disclosed is a method using a supercritical fluid (for example, Patent Document 1). It is also conceivable to take a method of introducing the resin into a degasifying tank of a high temperature to remove the volatile component. This method is a method in which the resin is passed, after the polymerization therefor, into the high-temperature degasifying tank. Thus, a machine other than an extruder is required, so that a bad efficiency is unavoidable. In the meantime, about a product by imidizing an acrylic resin in order to improve the heat resistance of polymethyl methacrylate, Patent Document 2 describes a method of degasifying the resin under the atmospheric pressure or in a vacuum when the resin is extruded, thereby removing amine remaining in the resin; and Patent Document 3 describes a method of degasifying the resin again, together with methanol or water, and also describes a method of extruding the resin in a high-degree vacuum under a high screw-rotation as a method for removing oligomer components. As a method for removing the volatile component, it is conceivable to increase the number of times of passing for extruding pellets for molding, or to extrude the resin at a high temperature under a high shear. This method promotes a deterioration or decomposition of the acrylic resin. About, in particular, easily decomposable or reactive copolymer resin, this method cannot be applied thereto since a fatal bad effect is produced onto the quality of a molded body therefrom. Any one of the above-mentioned methods is a method of removing any volatile component in acrylic resin forcibly after polymerization therefor, thereby reducing, about the yielded acrylic resin, the generated-gas-quantity at the time of molding the resin. The method is not a method in which at the stage of the polymerization for the acrylic resin, an aim is directed to a reduction in the generated-gas-quantity at the molding process time.
In the meantime, out of acrylic resins, polyfluoroalkyl(meth)acrylate, or a copolymer made mainly of fluoroalkyl(meth)acrylate has a low refractive index as an optical property, and is positioned as an especial acrylic polymer which has water-repellent and oil-repellent surface property, and which is excellent in radioactive ray sensitivity based on a characteristic solubility of the polymer, hygroscopicity, and dimension stability. An example of the application of such a fluoroalkyl(meth)acrylate polymer to an industrial material is application to a material constituting a light-conducting body. As a core material thereof, polystyrene or polymethyl(meth)acrylate is used, and as a sheath material thereof, a fluoroalkyl(meth)acrylate polymer is used, which is lower in refractive index than the core material. In recent years, attention has been paid to a contamination-resistant film about which notice is taken of the water-repellent and oil-repellent surface property that fluororesins have. Of the resins, polyvinylidene fluoride has been used for a melt-moldable member for vehicles that is an alternative to coating. About fluoroalkyl(meth)acrylate polymer also, investigations have been made about a matter that while good use is made of properties that the resin is an amorphous resin and is high in transparency, the polymer is utilized as a vehicle interior member for decorating plastic surfaces.
As described above, as functional material, attention has been paid to fluoroalkyl(meth)acrylate polymer since the polymer has characteristic material properties. However, as any ester of (meth)acrylic acid esterified with a fluorinated alcohol is compared with any ester thereof esterified with an alcohol wherein the same carbon atom(s) (as in the fluorinated alcohol) is/are not fluorinated, the former ester is lower in the density of double bonds, which have radical polymerizability, and has a molecular structure that is more easily radical-depolymerized. This matter means that fluoroalkyl(meth)acrylate polymer is deteriorated whenever the polymer is thermally shaped repeatedly, so that the polymer is declined in polymerization degree, and is further deteriorated in physical property by plasticizing-effect of a fluorinated acrylic monomer generated after the radical-depolymerization. As described herein, fluoroalkyl(meth)acrylate polymer is low in thermal stability, and further the material thereof, which is a fluoroalkyl(meth)acrylate monomer, is expensive; therefore, product-development thereof has been made, up to the present time, mainly about products based on painting or coating, which are not easily deteriorated by heat. Thus, fluoroalkyl(meth)acrylate polymer has not been sufficiently developed about applied products based on melt-molding process, and has not been sufficiently researched about the molding process. As a countermeasure against the heat deterioration, a method of the addition of a deterioration preventive is disclosed. However, the addition amount thereof is required to be larger (for the resin) than for ordinary acrylic resin, so that the resultant molded body may be deteriorated in transparency; or a problem is caused about the compatibility of the fluororesin that is based on water-repellent and oil-repellent property peculiar to this resin, and this problem may cause bleeding-out, resulting in a deterioration in the external appearance of the molded body. Patent Document 4 discloses an improvement of fluoroalkyl(meth)acrylate polymer in thermal stability by a method of adding a mercaptan having a mercapto group thereto. However, the matter that this resin, fluoroalkyl(meth)acrylate polymer, is low in thermal stability is still a cause of making it difficult to subject the polymer to melt-molding process, and develop the application thereof.